Electronically controlled variable gain amplifier



Jan. 6, 1948. HAYNES 2,434,155

ELECTRONICALLY CONTFKOLLED VARIABLE GAIN AMPLIFIER Filed Sept. 27, 1 9432 Sheets-Sheet 1 Boss/er 1}. H21 YNES, INVENTOR.

ATTORNEY.

Jan. 6, 1948-.

R. L. HAYNES 2 I Sheets- Sheet 2 Filed Sept. 27, 1943 I mm Mm m m v g77% mm 2% u E 3w w v HR s3 w? m @m QM QM sm m s .ww WW WW. WW. Wm W W aW M. w QM vhmm H W wwvwwawww m; n t: n u d H .1 H H n m $$$$$$R$R$$Mwkmmw 4 R R a. R R R R R \m \m w M mm mm mm mm mm mm mm mm mm. mm mm. QM Q3% Q g Patented Jan. 6, 1948 ELECTRQNICALLY CONTROLLED VARIABLE GAINAMPLIFIER Robert L. Haynes, Indianapolis, Ind., assignor to RadioCorporation of America, a corporation of Delaware [application September27, 1943, Serial No. 503,971

7 Claims.

This invention relates to electrical current transmission systems andparticularly to a system wherein the recorded or reproduced volumes ofthe signals are varied with respect to the original volumes during thetransmission of the signals.

The use of compression and expansion in sound recording, broadcast,public address, and other types of sound reproducing systems, iswellknown, a typical sound recording compressor system being disclosedand claimed in Singer Patent No. 2,255,683 of September 9, 1941. MillerPatent No. 2,136,723 of November 15, 1938, also discloses and claimsboth a compression and expansion system for sound films.

The present invention is directed to an improvement in compression andexpansion systems, and utilizes, to some extent, the principle disclosedin Strieby Patent No. 1,776,822 of September 30, 1930. The improvementover these prior types of volume control systems results in a loweramount of distortion, a very low thump disturbance, a simplification ofthe usual critical balancing procedure, and the avoidance of speciallyselected vacuum tubes. Thus, the compressor described hereinafterprovides these new results together with a smooth variation in volumedifferential over the range desired.

The principal object of the invention, therefore, is to facilitatecompression and expansion of the amplitudes of electrical currentscorresponding to sound signals.

Another object of the invention is to provide means for obtainingvariations in attenuation or loss in an electrical transmission systemin accordance with the original volume of the signal.

A further object of the invention is to provide means for varying theattenuation of a signal in accordance with its volume with the minimumof distortion and thump disturbance.

A still further object of the invention is to provide a balanced,electrically variable attenuator whose balance is not critical.

Although the novel features which are believed to be characteristic ofthis invention will be pointed out with particularity in the appendedclaims, the manner of its organization and the mode of its operationwill be better understood by referring to the following description readin conjunction with the accompanying drawings forming a part hereof, inwhich:

Fig, 1 is a schematic circuit diagram il1ustrating a fundamentalprinciple of the invention.

Fig. 2 is a graph illustrating the operation of the circuit of Fig. l.

Fig. 3 is a schematic circuit diagram illustrating the method ofapplying the basic principle of the present invention.

Fig. 4 is a graph illustrating the operation of the circuit of Fig. 3,and,

Fig. 5 is a schematic diagram of a compressor circuit embodying theinvention.

Referring now to Fig. 1, a plurality of diodes 5 are shown connected inseries with a respective number of resistors 6, all of which are inshunt to a resistor 3 across terminals 9 and iii. Each of the diodes isdifierently biased by respective batteries l2 having polarities asindicated. With such a circuit, small values of volta e applied toterminals 9 and ill with the polarity shown,

result in a resistance which is equal to the re.

sistance of resistor 8, since all of the diodes 5 have a, negative platevoltage and do not conduct. As the applied voltage across terminals Qand Ill is increased, the first diode begins to conduct when its platevoltage becomes positive and its resistor 5 is then shunted acrossresistor 3. As the applied voltage increases, the other diodes will besuccessively shunted across resistor 8, the effective resistance acrossthe line becoming lower and lower as the applied voltage is increased.As the normal diode rectifier has a rounded cut-0d characteristic, thecurve for the plurality of rectifiers 5 will appear as shown in Fig. 2,three different values of bias a, b, and 0 being shown on the curveplotted between applied voltage and current.

Although the above circuit may be used in a variable attenuator, such anattenuator will have several disadvantages, such as the introduction ofsecond harmonic distortion and the produc tion of objectionable thump inthe output when the ain or loss in the system is changed rapidly. Thesedisadvantages, however, may be eliminated or, at least, minimized to atolerable extent, by using two identical diode elements connected in apush-pull relationship and ing them equally but with oppositepolarities. Such a circuit is shown in Fig. 3 shunting terminals 9 andHi as in Fig. 1. In the circuit of Fig. 3, resistor l i' corresponds toresistor 8 of Fig. 1 and determines the hi hest resistance to theapplied voltage which is effective for small values thereof: Theremainder of the circuit comprises a plurality of diodes 5 with theirrespective resistors it similar to those of Fig. l, and a secondidentical plurality of diodes IS in series with their respectiveresistors 19, all of which are in shunt to the line having terminals 9and III. A polarizing potential is connected across terminals 2| and aseries of biasing batteries 22 provide a successively higher bias foreach pair of diodes l and IS.

The voltage-current curves for the circuit of Fig. 3 are shown in Fig. 4for three different biases d, e, and 1, curve 11 being for a low valueof bias and curves e and I being for successively higher values of bias,the effective resistance of the combination to small audio signals beingindicated by the slope of the curves at the origin. From these curves,it will be noted that the effective resistance may be varied over a widerange by shifting the bias. The use of two identical elements, such asthe respective pairs of diodes l5 and I8 and the respective resistors l6and i8 used in the manner shown in Fig. 3, prevents the objectionablethumps and second harmonic distortion. It is true that third harmonicdistortion may be present, but this distortion is of a very lowamplitude and may be further minimized by keeping the audio signal levellow.

The above type of volume control circuits are of the non-linear type, asshown by the curves in Figs. 2 and 4, and some of the disadvantages ofsuch circuits are that they usually must be perfectly balanced ormatched to avoid distortion, that their characteristics generally varywith ambient temperature, and that they introduce distortion because ofthe attenuation at high frequencies due to the high capacitance in theelements themselves. My arrangement of these tubes in the push-pullrelationship shown in Fig. 3, however, reduces these disadvantages to aminimum and reduces the tolerance to which the balance must be held,both of the tubes and of their series resistors.

Referring now to Fig. 5, wherein is shown an actual compressor circuitarrangement, it being understood that an expander utilizes the sameprinciple in reverse, the incoming signal to be compressed is impressedacross terminals 25 connected to an incoming line and transmitted overtransformer 23 to the line 29 and 30, between which are located shuntingcircuits made up of a plurality of diodes 3| with their respectiveseries resistors 32 and a like number of diodes 34 with their respectiveseries resistors 36. It will be noted that this circuit arrangement issimilar to the arrangement of diodes and resistors in Fig. 3, thebatteries 22 of Fig. 3 now being replaced by bias resistors 37. In thecircuit of Fig. 5, the diodes 3i and 34 with their respective resistors32 and 36 form the variable shunt resistor of an H-pad, resistors Ell,4i, 42, and 43 forming the series elements of the pad. Resistors #1 a5.and 455 form the terminating resistance for the secondary of transformer26, while resistor 41 performs a similar function for the primary ofoutput transformer 49.

After the electrical currents are transmitted by transformer 43, theyare amplified by an alternating current amplifier 58, the output ofwhich, after passage through coupling condenser 14, is divided betweenthe output transformer 52, the output terminals of which are connectedto an outgoing line connected to the load, and the couplingpotentiometer resistor Varying amounts of the voltage developed acrossresistor 53 are impressed upon the second alternating current amplifier55, the output being impressed upon a transformer 56, the secondary ofwhich is connected to a pair of high voltage rectifiers 58 and a pair oflow voltage rectifiers 59. This particular type of rectifier system isdisclosed and claimed in my United States Patent No. 2,359,989 ofOctober 10, 1944. The output of the rectifiers 58--59 is impressed on adirect current amplifier 6! which controls the over-all impedance acrossthe line 29--30. Variable resistor 64 controls the initial bias appliedto the diode circuit, while the system includes the usual by-passcondensers 65, 66, 87, 68, and 69 and bias resistors 10 and H. Couplingcondensers are shown at I3 and I4, while anode load resistors are shownat 15, I3, and 11.

As mentioned above, the above system operates as a compressor since theattenuation elements of the H-pad, comprising the fixed series resistors49, 4|, 42, and 43, and the variable shunting resistor composed of theplurality of diodes 3| and 34 with their associate resistors 32 and 36,is varied in accordance with the amplitude of the signal acrossterminals 25. Suitable different bias voltages are provided for eachpair of the diodes 3| and 34 by the respective resistors 3'! in thevoltage divider circuit connected across the 250-volt plate potentialsupply. The attenuation or loss introduced by the pad is determined bythe voltage existing between terminals 62 and 63. When terminal 62 isnegative with respect to terminal 63, or when the difference between thepotential at terminals 62 and 63 is zero, none of the diodes isconducting and the effective value of the shunt impedance of the H-padis infinite, which provides the minimum loss condition. In oneparticular circuit, the loss thus introduced was about 14 db. When theterminal 62 is 70 volts positive, with respect to terminal 63, all thediodes are conducting and all of the resistors 32 and 36 are effectiveas shunt resistors in the H-pad, and, in one type of circuit, this shuntimpedance was about 2000 ohms introducing a loss of 34 db. Atintermediate values of control voltage, only certain of the diodes areconductive, and the loss was smoothly variable between 14 and 34 db. Inthis circuit, the bias on the successive diodes differed by aboutone-half a volt and the individual diode plate characteristics weresufficiently curved to produce a smooth variation of gain with controlvoltage with no abrupt amplitude changes.

The above-described compressor was found to be an improvement over priorcompressors in that it had a particularly low transmission distortion, alower thump disturbance, and was less critical to unbalance, thus notrequiring specially selected tubes. The circuit does not drift offbalance, thus reducing readjustments. About 20 db. of compression wasobtainable, the shape of the compression curve being determined by thevalue of resistors 53 and B4, resistor 64 determining the initial biasapplied to the diode circuit which, of course, determines the point atwhich compression starts.

Although my special rectifier system is illustrated at 58-439, it is tobe understood that any standard full wave filter-rectifier circuit maybe substituted, although the rectifier circuit shown in Fig. 5 ispreferred since it was found to give approximately a 6 to 8 db.improvement in filtering with no sacrifice in timing characteristics.

I claim as my invention:

1. An electrical current transmission system comprising an incoming lineand an outgoing line, a resistor connected in shunt to said line fordetermining the maximum impedance across said line, and a plurality ofpairs of diodes connected in shunt across said line for reducing theimpedance across said line below that of said resistor, the diodes ofeach pair having their cathodes connected together, said diodes beingarranged in pairs and each pair being connected in parallel and providedwith successively higher biases for successively decreasing theimpedance across said line in accordance with the voltage appliedthereto.

2. An electrical current transmission system comprising an incoming lineand an outgoing line, an impedance network interconnecting said lines,said network comprising a plurality of series resistors and anintermediate variable impedance element, said variable impedance elementincluding a plurality of diodes arranged in pairs in shunt to said line,the diodes of each pair having their cathodes connected together, meansfor obtaining a difierent bias for each pair of diodes, and means forapplying a voltage to the anodes of said diodes simultaneously inaccordance with the amplitude of the signal present in said incomingline for varying the impedance across said line.

3. An electrical transmission system in accordance with claim 2 in whichsaid last mentioned means includes a rectifier for producing a voltagecorresponding to the peak value of the amplitude of said signalcurrents, said voltage being impressed simultaneously on all the anodesof said diodes.

4. A compressor circuit comprising an incoming line for signal currents,an outgoing line for said currents, and a variable impedanceinterconnecting said lines, said variable impedance including aplurality of diodes connected in pairs across said line, the diodes ofeach pair having their cathodes connected together and each pair ofdiodes being connected in parallel, means for obtaining a different biason each parallel pair of diodes, and means for varying the anodepotential on all of said diodes simultaneously for varying theattenuation of said signal currents before impression on said outgoingline in accordance with the peak value of the amplitude of said signalcurrents.

5. A compressor circuit in accordance with claim 4 in which said lastmentioned means includes a rectifier having its input connected to theoutput of said variable impedance and its output connected to the anodepolarizing circuit for said plurality of diodes 6. An electricalcompressor circuit comprising an incoming signal line and an outgoingsignal line and a network interconnecting said lines for varying theamplitude of transmission of the signal currents between said lines,said network including a variable electronic impedance including aplurality of pairs of diodes, the diodes of each pair having theircathodes connected together, each pair of diodes being connected inparallel and having a successively higher bias, and means forsimultaneously varying the anode potential on said diodes in accordancewith the peak value of the amplitude of said signal currents.

7. An electrical compression circuit in accordance with claim 6 in whichsaid network is in the form of an H-pad having a plurality of seriesresistors, said plurality of diodes forming the shunt arm of sair H-pad.

ROBERT L. HAYNES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,208,923 Curtis July 23, 19402,215,946 Von Radinger Sept 24, 1940 2,298,657 Smith et a1 Oct. 13, 1942FOREIGN PATENTS Number Country Date 478,137 Great Britain Jan. 10, 1938

